In a typical swash-plate type variable capacity compressor, the tilt angle of a swash-plate can be controlled by the change in the pressure of the refrigerant which is controlled by the pressure control valve depending on the thermal load. By controlling the tilt angle of a swash-plate, the stroke distance of the piston is changed thereby changing the discharge capacity of refrigerant and thus controlling the temperature of an evaporator.
The pressure control valve can be either internal-controlling type or external-controlling type, and a typical structure of swash-plate type variable capacity compressor is disclosed in Japanese laid open patent application No. 2001-107854.
The output to the pressure control valve (duty: current value) can be controlled by calculating the deviation between the target evaporator temperature and actual evaporator temperature and carrying out proportional-integral (PI) control based on this deviation. For example, Japanese laid open patent application No. 2003-200730 discloses a method of controlling air conditioner wherein proportional-integral control is carried out when the deviation between the target evaporator temperature and actual evaporator temperature is less than 3° C., maximum capacity control is carried out when the actual evaporator temperature is higher than the target evaporator temperature by not less than 3° C., and minimum capacity control is carried out when the actual evaporator temperature is lower than the target evaporator temperature by not less than 3° C.
In carrying out proportional-integral control by feedbacking actual evaporator temperature to the target evaporator temperature, the temperature controlling performance, i.e. the temperature convergence and response time, depends on how properly the proportional gain and integral gain is set depending on the system. When gain becomes larger, response time is shortened but system stability can be deteriorated due to excessive occurrence of overshoot and undershoot. On the contrary, when gain becomes smaller, temperature convergence is enhanced but the time for stabilization becomes longer.
From these points, the method of prior arts carrying out maximum or minimum capacity control with a predetermined fixed value even when the deviation between the target evaporator temperature and the actual evaporator temperature is over-higher or over-lower than a preferable value has the problem of causing instability in evaporator temperature and system.